Any scratching/flashing on objects in the tank or sitting at the bottom? Also, look for any white hairy patches on the skin. Clamped fins can be a symptom for many different problems. Parasites, bacterial infections, finrot, fungal infections, or even a buildup of high nitrates (you could be getting a false reading as this can happen a lot), temp or pH of/in the water can cause this. I would suggest doing a 20% water change to see if he improves and be sure to let us know if you see any other symptoms. Also, what kind of test kit are you using?